High voltage generators or power supplies for producing DC voltages which may lie in ranges from tens of kilovolts to megavolts, are used in a number of applications in various fields of technology. Typical output currents lie in the range from microamperes to hundreds of milliamperes. For example, such high voltage sources may be used in providing power for X-ray generators, nuclear accelerators, electrostatic precipitators, sophisticated medical electronic devices, such as CT scanning equipment, and the like. Well-known high voltage sources of the prior art which have found use in many applications include transformer power supplies, Van de Graaff generators, and rectifier multiplier chain circuits often designated as Cockroft-Walton generators or Greinacher circuits. Later modifications of the latter circuits use a dual chain of rectifier/multiplier circuits, wherein each stage comprises a bridge rectifier, the rectifiers thereof being separated by capacitances. The input is supplied by an R-F source via a transformer having a center-tapped secondary, the ends of the secondary being connected to the side chains of circuit capacitors and the center tap thereof being connected to the center chain of capacitors.
One of the problems with the original Greinacher circuit, and somewhat less pronounced in dual chain configurations thereof, is that the effective internal impedance is relatively high and increases as the number of stages thereof increases (essentially as the cube of the number of stages). A high effective internal impedance produces an undesirable voltage loss along the circuit severely limiting the power output of the supply. All rectifier circuits, and particularly multiplier circuits, produce undesirable ripple voltages (the ripple being essentially proportional to the square of the number of stages). As the number of stages increases, the capacitance values must be increased to reduce the ripple as well as to keep the overall voltage drop low. The need for relatively large capacitors means that the stored energy in the circuit cannot be minimized, as is desirable in such high voltage circuits. Moreover, should the load be temporarily shorted out, as sometimes occurs in the environment in which such voltage supplies are used, a large amount of stored energy will be substantially immediately released to produce relatively powerful sparks with attendant substantial damage.
Attempts to overcome the disadvantages of conventional rectifier/multiple circuit arrangements have been made, as shown in U.S. Pat. No. 3,505,608 issued on Apr. 7, 1970 to H. A. Enge and U.S. Pat. No. 3,543,136 issued on Nov. 24, 1970, such devices often being referred to as transmission-line generators and in U.S. Pat. No. 3,596,167 issued on July 27, 1971 to H. A. Enge and sometimes referred to as the Deltatron (a trademark of High Voltage Engineering Corporation, Burlington, Mass.) configuration.
Both types of high voltage supplies utilize a primary source of moderately high frequency (typically 50-100 kHz). Both supplies transport power along a stack of inductor-capacitor units by inductive coupling between coils in each unit. In each unit part of the power is withdrawn and rectified to produce DC outputs connected in series from unit to unit.
A principal reason for using a high-frequency primary power source of high-voltage generation is to reduce the bulk and expense of the components, capacitors and inductors in the rectifier unit or units. Moreover, a high frequency supply has a relatively short response time and is, therefore, relatively easy to regulate. For instance, a one-million volt Deltatron unit which has been built has a stability of better than one part in 100,000. Finally, since it is important in some applications, at least, to minimize the stored energy in the high voltage generator (e.g., when high voltage breakdown is likely to occur in the load), the energy extractable from the power source under breakdown conditions should preferably be small. The use of smaller capacitors in such high frequency units reduces the extractable stored energy.
One of the problems in the transmission line configuration of the aforesaid U.S. Pat. No. 3,505,608 which uses a traveling wave is that, in order to avoid reflections producing a standing wave, the transmission line must be terminated by its characteristic impedance or the traveling wave must be transmitted in the reverse direction along a separate chain to ground. In the first case a relatively large amount of power is wasted and excessive heating is produced in the terminating resistance. In the second case the extra chain of inductors, capacitors, and possibly also rectifiers, doubles the complexity of the device.
The transmission line configuration of the aforesaid U.S. Pat. No. 3,543,136 attempted to overcome the disadvantages of traveling wave transmission line generators by using a standing wave and withdrawing the power at or close to the antinodes. However, if the standing wave configuration is in the form of a relatively long chain or stack (i.e., a large number of stages), the tolerance requirements for the electrical components become excessively stringent.
In the aforesaid Deltatron generator a plurality of air-core transformer stages are capacitively coupled in a cascade configuration and are connected to rectifier-multipliers, also formed in a chain, so as to produce the overall DC voltage output. The capacitance values which couple the various stages must be appropriately selected to provide both series and parallel resonance conditions. The overall configuration thereby becomes very critical in its tuning capabilities. In many applications the need for such critical tuning becomes impractical to achieve, particularly for a relatively long stack.
It, therefore, becomes desirable to provide a simpler solution to the problems that arise in the basic Cockroft-Walton or Greinacher rectifier-multiplier chain circuitry and in the alternative configurations heretofore suggested by those in the art. Such solution must provide an adequate voltage output (i.e., adequate power) at a relatively reasonable cost using a relatively simple circuit configuration which can be readily fabricated.